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1.
A linear zone with high strain rates along the Japan Sea coast, the Niigata-Kobe Tectonic Zone (NKTZ), is considered to be associated with rheological heterogeneities in the lower crust and/or upper mantle. Helium isotope variations along the NKTZ reveal a close association with the geophysical evidence for rheological heterogeneities in the crust and mantle. In the southern NKTZ, the 3He/4He ratios lower than 3.4 Ra (Ra denotes the atmospheric 3He/4He ratio of 1.4 × 10−6) could be interpreted as a two-component mixture of helium stored in aqueous fluids driven off the subducting oceanic crust and radiogenic crustal helium. Higher 3He/4He ratios are observed in the central NKTZ where Quaternary volcanoes and high-temperature hot springs are concentrated, suggesting that the 3He emanation manifest in the central NKTZ results from the effective transfer of mantle helium by intrusion and degassing of mantle-derived magma in the crust. In the northern NKTZ where two large inland earthquakes occurred recently, there appears to be many samples with 3He/4He ratios significantly higher than those observed in the fore-arc side of northeast Japan. A plausible source of mantle helium could be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab.  相似文献   

2.
We investigate the helium, carbon and oxygen–hydrogen isotopic systematics and CO2/3He ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western Anatolia (Turkey). 3He/4He ratios of the samples (R) normalized to the atmospheric 3He/4He ratio (RA = 1.39 × 10? 6) range from 0.27 to 1.67 and are significantly higher than the crustal production value of 0.05. Fluids with relatively high R / RA values are generally found in areas of significant heat potential (K?z?ldere and Tuzla fields). CO2/3He ratios of the samples, ranging from 1.6 × 109 to 2.3 × 1014, display significant variation and are mostly higher than values typical of an upper mantle source (2 × 109). The δ13C (CO2) and δ13C (CH4) values of all fluids vary from ? 8.04 to + 0.35‰ and ? 25.80 to ? 23.92‰ (vs. PDB), respectively. Stable isotope values (δ18O–δD) of the geothermal waters are conformable with the Mediterranean Meteoric Water Line and indicate a meteoric origin. The temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers, implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface.Evaluation of He–CO2 abundances indicates that hydrothermal degassing and calcite precipitation (controlled probably by adiabatic cooling due to degassing) significantly fractionate the elemental ratio (CO2/3He) in geothermal waters. Such processes do not affect gas phase samples to anywhere near the same extent. For the gas samples, mixing between mantle and various crustal sources appears to be the main control on the observed He–C systematics: however, crustal inputs dominate the CO2 inventory. Considering that limestone is the main source of carbon (～ 70 to 97% of the total carbon inventory), the carbon flux from the crust is found to be at least 20 times that from the mantle. As to the He-inventory, the mantle-derived component is found to vary up to 21% of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension. The range of 3He/enthalpy ratios (0.000032 to 0.19 × 10? 12 cm3 STP/J) of fluids in western Anatolia is consistent with the release of both helium and heat from contemporary additions of mantle-derived magmas to the crust. The deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface.  相似文献   

3.
The Maevatanana gold deposit in Madagascar is hosted by Archean metamorphic rocks in quartz–sulfide veins that are structurally controlled by NNW–SSE trending shear zones. Fluid inclusion data show that the trapping conditions in quartz range from 0.87 to 2.58 kbar at temperatures of 269–362 °C. Laser Raman spectroscopy confirms that these inclusions consist of CO2, SO2, and H2O. The δ34S values of the pyrites range from 1.7‰ to 3.6‰, with an average of 2.25‰, supporting a magmatic origin. Noble gases (He, Ne, Ar, Ke, Xe) are chemically inert, thus will not be involved in chemical reactions during geological processes. Also due to the low concentration of He in the atmosphere and the low solubility of He in aqueous fluids, the atmosphere-derived He is unlikely to significantly affect He abundances and isotopic ratios of crustal fluids, ensures that He production should have the typical crust 3He/4He ratios. The 3He/4He ratios of fluid inclusions in pyrite from the deposit range from 0.06 to 0.12 Ra, while the 40Ar/36Ar ratios range from 6631 to 11441. We infer that the ore-forming fluids could have been exsolved from a granitic magma. The oxygen and hydrogen isotope compositions of the ore-forming fluids (1.5‰ ≤ δ18OH2O ≤ 7.8‰; –72‰ ≤ δD ≤ –117‰) indicate they were derived from a granitic magma. Four pyrite samples from the gold deposit yield a precise Re–Os isochron age of 534 ± 13 Ma. Given that the post-collisional granites in northern and central Madagascar were derived by melting of sub-continental lithospheric mantle and formed between 537 and 522 Ma, we can state that the gold metallogenesis was coeval with the crystallization age of these parental magmas. These data could be accounted for the formation of the Maevatanana gold deposit. First, the shear zones hosting the deposit formed around 2.5 Ga, when the Madagascan micro-continental blocks collided with other continental blocks, triggering large-scale tectono-magmatic activity and forming NNW–SSE trending shear zones. The gold mineralization at Maevatanana is coeval with the crystallization age of the Cambrian post-collisional A-type granitoid plutons in northern and central Madagascar, implying that this deposit is associated with extensional collapse of the East African Orogen. This extension in turn induced asthenospheric upwelling, melting of sub-continental lithospheric mantle. These magmas underplated the lower crust, generating voluminous granitic magmas by partial melting of the lower crust. The mixing magma during tectono-thermal reactivation of the East African Orogen produced large volumes of volatiles that extracted gold from the granitic magma and produced Au–S complexes (e.g., Au(HSO3)2−). The shear zones, which were then placed under extensional collapse of the East African Orogen in the Cambrian, formed favorable pathways for the magmatic ore-forming fluids. These fluids then precipitated gold-sulfides that form the Maevatanana gold deposit.  相似文献   

4.
We analyzed major and trace elements, Sr and Nd isotopes in ultramafic xenoliths in Miocenic age Hyblean diatremes, along with noble gases of CO2-rich fluid inclusions hosted in the same products. The xenoliths consist of peridotites and pyroxenites, which are considered to be derived from the upper mantle. Although the mineral assemblage of peridotites and their whole-rock abundance of major elements (e.g., Al2O3 = 0.8–1.5 wt.%, TiO2 = 0.03–0.08 wt.%) suggest a residual character of the mantle, a moderate enrichment in some incompatible elements (e.g., LaN/YbN = 9–14) highlights the presence of cryptic metasomatic events. In this context a deep silicate liquid is considered the metasomatizing agent, which is consistent with the occurrence of pyroxenites as veins in peridotites. Both the Zr/Nb and 143Nd/144Nd ratios of the investigated samples reveal two distinct compositional groups: (1) peridotites with Zr/Nb ≈ 4 and 143Nd/144Nd ≈ 0.5129, and (2) pyroxenites with Zr/Nb ≈ 20 and 143Nd/144Nd ≈ 0.5130. The results of noble-gas analyses also highlight the difference between the peridotite and pyroxenite domains. Indeed, the 3He/4He and 4He/40Ar* ratios measured in the fluid inclusions of peridotites (respectively 7.0–7.4 ± 0.1 Ra and 0.5–8.2, where Ra is the atmospheric 3He/4He ratio of 1.38 × 10? 6) were on average lower than those for the pyroxenites (respectively 7.2–7.6 Ra and 0.62–15). This mantle heterogeneity is interpreted as resulting from a mixing between two end-members: (1) a peridotitic layer with 3He/4He ≈ 7 Ra and 4He/40Ar* ≈ 0.4, which is lower than the typical mantle ratio (~ 1–4) probably due to melt extraction events, and (2) metasomatizing mafic silicate melts that gave rise to pyroxenites characterized by 3He/4He ≈ 7.6 Ra, with a variable 4He/40Ar* due to degassing processes connected with the ascent of magma at different levels in the peridotite wall rock. The complete geochemical data set also suggests two distinct mantle sources for the xenolithic groups highlighted above: (1) a HIMU (high-μ)-type source for the peridotites and (2) a DM (depleted mantle)-type source for the pyroxenites.  相似文献   

5.
The North China Craton (NCC) provides a classic example for extensive destruction of the cratonic lithosphere. The Mesozoic magmatism which contributed to the decratonization of the NCC was also accompanied by the formation of a variety of mineral deposits. In order to gain further insights into the cratonic destruction process, typical iron and gold deposits are investigated here. Helium–argon isotopic data on pyrite, from typical skarn iron deposits of the Beiminghe and Fushan in the Han-Xing district of the central NCC, and the Linglong and Canzhuang gold deposits in the Jiaodong district in the eastern NCC, are presented in this paper. The 3He/4He, 40Ar/36Ar and 40Ar/4He ratios show generally uniform patterns within the individual deposits and reveal a complex evolutionary history of the ore-forming fluids with varying degree of crust–mantle interaction. The ore-forming fluids associated with the gold mineralization at the Jiaodong mine have higher content of fluids of mantle origin with mantle helium ranging from 1.24% to 18.02% (average 6.73%; N = 18). In contrast, the ore-forming fluids related to the iron ore deposits contain less mantle contribution with mantle helium ranging from 0.12% to 4.96% (average 1.29%; N = 10). Our results suggest complex and heterogeneous crust–mantle processes associated with the magmatism and metallogeny, where the lithosphere of the eastern NCC was subjected to more extensive thinning and destruction as compared with that in the western part, consistent with the observations from geophysical studies in the region. Our study demonstrates that fluids associated with the Mesozoic metallogenic processes in the NCC provide useful insights into the geodynamics of destruction and refertilization of the cratonic lithosphere.  相似文献   

6.
The large scale Mesozoic magmatism and related metallogeny in the Taihang Mountains (TM) provide important clues for the lithospheric thinning of the North China Craton (NCC). Among the ore deposits, the vein gold mineralization of Shihu in the Fuping region and the skarn ore deposit of Xishimen in the Wu'an region represent typical Mesozoic metallogeny in the TM. In the Shihu gold mine, the Mapeng batholith is dominantly composed of monzogranite and granodiorite, whereas, the Wu'an pluton in the Xishimen iron mine mainly comprises monzonite and diorite. Here we present zircon LA–ICP-MS U–Pb data from 8 samples which reveal the timing of magmatism in the TM as ca. 130 Ma, which is contemporaneous with the large-scale metallogeny in the margins of the NCC. The δ34S values recorded in the sulfide minerals from the Shihu gold deposit and the Xishimen skarn iron deposit show a range of 2.2‰–5.0‰, and 11.6‰–18.7‰, respectively. Helium isotopic compositions of fluid inclusions in pyrite from the Shihu gold deposit vary from 0.12 to 1.98 Ra (where Ra is the 3He/4He ratio of air = 1.39 × 10? 6), with calculated mantle helium values of 1.4%–25%, whereas, those of the Xishimen skarn iron deposit range from 0.06 to 0.19 Ra, with calculated mantle helium of 0.7%–2.2%. The S–He–Ar isotopic data suggest a lower crustal origin for the ore-forming components, with variable inputs of mantle source. The large population of inherited zircons in our samples, with 207Pb/206Pb ages ranging between 2500 Ma and 1800 Ma, also supports crustal participation. Our data reveal that the Shihu gold deposit witnessed greater mantle input than the Xishimen skarn iron deposit, suggesting that the continental lithosphere is markedly thinner under the Fuping region than that under the Wu'an region. Our interpretation is also supported by published data from two ultra-broadband high-precision magnetotelluric sounding profiles across the TM region showing a variation in the lithosphere thickness from 155 km to 70 km while moving from the south (Wu'an region) to the north (Fuping region). Our study suggests that inhomogeneous lithospheric thinning in the central NCC occurred at least as early as ca. 130 Ma ago.  相似文献   

7.
We present new helium isotope data from the Macdonald seamount (Austral chain). The helium isotopic ratio varies from 4He/3He=45 000 (R/Ra=16.0) to 200 170 (R/Ra=3.6). The helium content is between 1.5×10?8 and 1.1×10?5 ccSTP/g. These helium results show clearly the presence of primitive mantle material in the source of the Austral chain. Macdonald has the lowest 4He/3He ratio among the Polynesian submarine volcanoes, except Hawaii (Loihi). The simplest explanation for the primitive helium signature is the presence under Macdonald of a mantle plume that derives either from the 670 km or 2900 km boundary layers, or, eventually, from the top of a large mantle dome resulting from a stratified two-layer convection. This plume contains less-degassed material with low 4He/3He ratio. To cite this article: M. Moreira, C. Allègre, C. R. Geoscience 336 (2004).  相似文献   

8.
Static stress changes caused by megathrust slip of the 2011 Mw 9.0 Tohoku-Oki earthquake considerably affected the seismicity patterns in inland areas, resulting in the occurrence of numerous earthquakes along several active faults in Japan. On June 30, 2011, the Mj 5.4 central Nagano earthquake occurred at a shallow depth of 5 km, indicating the reactivation of the Gofukuji fault in Central Japan. This study was undertaken to elucidate spatial and temporal changes of 3He/4He ratios around a source region before and after an inland earthquake using both existing and new and helium isotope data from hot spring and drinking water wells. Gas samples near the Gofukuji fault and its surrounding active faults are characterized by an increase in postseismic 3He/4He ratios. In contrast, the postseismic ratios decreased by up to about 30% away from the mainshock epicenter. Episodic faulting could either release stored crustal (radiogenic) helium from host rocks, or enhance the transfer of mantle volatiles through permeable fault zones, such that subsequent fluid flow near to the source region could then explain the spatio-temporal variations in 3He/4He ratios.  相似文献   

9.
The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (> 200 km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (3He/4He) ratios (up to 9.1 Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing 3He/4He values in the SCLM range (3He/4He < 7 Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (YN and YbN, respectively) indicate that fractional partial melting might have affected the local mantle by < 3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of 4He/40Ar* (40Ar* is corrected for atmospheric contamination) ranges from 4.9 × 10− 4 to 3.6 × 10− 1, which is below the typical production ratio of the mantle (4He/40Ar* = 1–5); this range is however compatible with this fractional partial melting. The variable 3He/4He and 4He/40Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO2-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.  相似文献   

10.
Chemical and isotopic compositions have been measured for CO2-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed 3He/4He ratios (2–3 RA) and δ13C values of CO2 (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO2/3He ratios are in wide range of (0.4–97 × 109). Based on the apparent mixing trend in a 3He/4He and δ13C of CO2 diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has 3He/4He, δ13C and CO2/3He value of ∼3.2 RA, ∼−4.6‰ and ∼6 × 1010, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO2 from the adjacent Changbaishan volcanic area.  相似文献   

11.
We present a new regional model for the depth-averaged density structure of the cratonic lithospheric mantle in southern Africa constrained on a 30′ × 30′ grid and discuss it in relation to regional seismic models for the crust and upper mantle, geochemical data on kimberlite-hosted mantle xenoliths, and data on kimberlite ages and distribution. Our calculations of mantle density are based on free-board constraints, account for mantle contribution to surface topography of ca. 0.5–1.0 km, and have uncertainty ranging from ca. 0.01 g/cm3 for the Archean terrains to ca. 0.03 g/cm3 for the adjacent fold belts. We demonstrate that in southern Africa, the lithospheric mantle has a general trend in mantle density increase from Archean to younger lithospheric terranes. Density of the Kaapvaal mantle is typically cratonic, with a subtle difference between the eastern, more depleted, (3.31–3.33 g/cm3) and the western (3.32–3.34 g/cm3) blocks. The Witwatersrand basin and the Bushveld Intrusion Complex appear as distinct blocks with an increased mantle density (3.34–3.35 g/cm3) with values typical of Proterozoic rather than Archean mantle. We attribute a significantly increased mantle density in these tectonic units and beneath the Archean Limpopo belt (3.34–3.37 g/cm3) to melt-metasomatism with an addition of a basaltic component. The Proterozoic Kheis, Okwa, and Namaqua–Natal belts and the Western Cape Fold Belt with the late Proterozoic basement have an overall fertile mantle (ca. 3.37 g/cm3) with local (100–300 km across) low-density (down to 3.34 g/cm3) and high-density (up to 3.41 g/cm3) anomalies. High (3.40–3.42 g/cm3) mantle densities beneath the Eastern Cape Fold belt require the presence of a significant amount of eclogite in the mantle, such as associated with subducted oceanic slabs.We find a strong correlation between the calculated density of the lithospheric mantle, the crustal structure, the spatial pattern of kimberlites, and their emplacement ages. (1) Blocks with the lowest values of mantle density (ca. 3.30 g/cm3) are not sampled by kimberlites and may represent the “pristine” Archean mantle. (2) Young (< 90 Ma) Group I kimberlites sample mantle with higher density (3.35 ± 0.03 g/cm3) than the older Group II kimberlites (3.33 ± 0.01 g/cm3), but the results may be biased by incomplete information on kimberlite ages. (3) Diamondiferous kimberlites are characteristic of regions with a low-density cratonic mantle (3.32–3.35 g/cm3), while non-diamondiferous kimberlites sample mantle with a broad range of density values. (4) Kimberlite-rich regions have a strong seismic velocity contrast at the Moho, thin crust (35–40 km) and low-density (3.32–3.33 g/cm3) mantle, while kimberlite-poor regions have a transitional Moho, thick crust (40–50 km), and denser mantle (3.34–3.36 g/cm3). We explain this pattern by a lithosphere-scale (presumably, pre-kimberlite) magmatic event in kimberlite-poor regions, which affected the Moho sharpness and the crustal thickness through magmatic underplating and modified the composition and rheology of the lithospheric mantle to make it unfavorable for consequent kimberlite eruptions. (5) Density anomalies in the lithospheric mantle show inverse correlation with seismic Vp, Vs velocities at 100–150 km depth. However, this correlation is weaker than reported in experimental studies and indicates that density-velocity relationship in the cratonic mantle is strongly non-unique.  相似文献   

12.
The Nanling Range in South China is characterized by extensive Mesozoic magmatism and coeval nonferrous and rare metal mineralization. Huangshaping is a world-class Pb-Zn-W-Mo polymetallic skarn deposit in the central Nanling Range. Magmatic rocks occurring in this ore district include quartz porphyry, granite porphyry, granophyre, dacite porphyry, and aplite, with only the first three granitoids genetically associated with polymetallic mineralization. Most of the orebodies are constrained within the contact zones as skarn and veins between these granitic stocks and the carbonate wall rocks.Since the age of the quartz porphyry is still controversial, and studies of the dacite porphyry and aplite are absent, we focus on these magmatic rocks first. LA-ICP-MS zircon U-Pb dating suggests that the crystallization ages of the quartz porphyry, dacite porphyry, and aplite are 154.3 ± 1.9 Ma, 158.1 ± 0.8 Ma, and 148.4 ± 3.4 Ma, respectively. Combined with previously published age data, we infer the evolutionary sequence of magmatic rocks should be dacite porphyry → quartz porphyry → granite porphyry (granophyre) → aplite. The quartz porphyry, dacite porphyry, and aplite yield high contents of high field strength elements (Zr + Nb + Ce + Y = 255–440 ppm), high ratios of 10,000 × Ga/Al (2.6–3.2), and prominent depletions in Ba, Sr, Eu, P, and Ti, indicating their crustal affinities to A-type granites. They have negative εNd(t) values (−9.4 to −7.0) and high initial Pb isotopic ratios (206Pb/204Pbi = 18.307–18.644, 207Pb/204Pbi = 15.689–15.742, 208Pb/204Pbi = 38.589–38.986), suggesting that they were probably derived by partial melting of ancient granulitic crustal materials.The sulfide minerals exhibit a wide range of δ34SV-CDT values from −22.6 to 24.2‰, with 206Pb/204Pb of 17.669–19.708, 207Pb/204Pb of 15.492–15.714, and 208Pb/204Pb of 37.880–39.789, indicating that sulfur, lead, and other associated metals were derived from a mixture of magmatic components and the Carboniferous wall rocks. Fluid inclusions in pyrrhotite, sphalerite, and marmatite samples have 3He/4He ratios of 0.12 to 1.53 Ra, with calculated mantle helium proportions of 1.3 to 18.9%, indicating a predominantly crustal origin for the ore fluids, with minor inputs from the mantle. The Huangshaping deposit is a typical example of the genetic relationship both spatially and temporally between Jurassic magmatism and polymetallic metallogeny in the Nanling Range.  相似文献   

13.
The Zhazixi Sb–W deposit in the Xuefeng uplift, South China, exhibits a unique metal association of W and Sb, where the W orebodies are hosted by interlayer fractures and the Sb orebodies are contained within NW-trending faults. This study proposes that the W and Sb mineralization took place in two separate periods. The mineral paragenesis of the W mineralization reveals a mass of quartz, scheelite and minor calcite. The mineral assemblage of the Sb mineralization developed after W mineralization and consists of predominantly quartz and stibnite, and small amounts of native Sb, berthierite, chalcostibnite, pyrite, and chalcopyrite. Fluid inclusions in quartz and coexisting scheelite are dominated by two-phase, liquid-rich, aqueous inclusions at room temperature. Microthermometric studies suggest that ore-forming fluids for W mineralization are characterized by moderate temperatures (170–270 °C), low salinity (3–7 wt% NaCl equiv.), low density (0.75–0.95 g/cm3), and moderate to high pressure (57.2–99.7 MPa) and these fluids experienced a cooling and dilution evolution during W mineralization. Ore-forming fluids for Sb mineralization are epithermal types with low temperatures (150–230 °C), low salinity (4–6 wt% NaCl equiv.), moderate density (0.82–0.94 g/cm3), and high pressure (42.2–122.5 MPa) and these fluids display an evident decline in homogenization temperature during Sb mineralization. Laser Raman analyses of the vapor phase indicate that the ore-forming fluids for both W and Sb mineralization contain a small amount of CO2.The ore-forming fluids for Sb mineralization are identified as predominantly originating from the continental crust, as suggested by the low 3He values (0.009 × 10−12 cc.STP/g) and 3He/4He ratios (0.002–0.056 Ra) as well as high 36Ar values (1.93 × 10−9 cc.STP/g) and 40Ar/36Ar ratios (909.5–2279.7). The source of S is identified to be the Neoproterozoic Wuqiangxi Formation, as traced by the δ34SV-CDT values of stibnite (3.1–9.4‰). The 208Pb/204Pb (37.643–40.222), 207Pb/204Pb (15.456–15.681), and 206Pb/204Pb (17.093–20.042) ratios suggest a mixture of lower crustal and supracrustal Pb sources.It is thus concluded that the ore genesis of the Zhazixi Sb–W deposit is related to the intracontinental orogeny during the early Mesozoic. Fluid mixing is considered to be the critical mechanism involved in W mineralization, whereas a fluid cooling process is responsible for Sb mineralization. Furthermore, the absence of Au is attributed to the low Σas content in Sb-mineralizing fluids.  相似文献   

14.
《Gondwana Research》2014,25(3-4):1108-1126
Detailed petrology and zircon U–Pb dating data indicate that the Wulong pluton is a zoned granitic intrusive, formed from successive increments of magmas. An age range of at least 30 Ma is recorded from the 225–235 Ma quartz diorite on the pluton margin, the ca. 218 Ma granodiorite in the intermediate zone, and the ca. 207 Ma monzogranite at the pluton center. All the granitoids display evolved Sr–Nd–Pb isotopic compositions, with 87Sr/86Sr(i) of 0.7044–0.7062, unradiogenic Nd (εNd(t) values of − 6.1 to − 3.0, Nd model ages of 1.1–1.3 Ga, and moderately radiogenic Pb compositions (206Pb/204Pb(i) = 17.500–17.872, 207Pb/204Pb(i) = 15.513–15.549, 208Pb/204Pb(i) = 37.743–38.001), in combination with variations in zircon Hf isotopic compositions (with εHf(t) values in each stage span 12 units) and the Hf isotopic model ages of 800–1600 Ma. These features suggest that the granitoids might have been derived from the reworking of an old lower crust, mixed with Paleozoic and Proterozoic materials. The rocks also display an adakitic affinity with Sr (479–973 ppm), high Sr/Y ratios (mostly > 60) and negligible Eu anomalies (Eu/Eu* = 0.78–0.97) but low Rb/Sr ratios, low Y (4.6–17 ppm), HREE (Yb = 0.95–1.7 ppm), Yb/Lu (6–7) and Dy/Yb (1.9–2.4) ratios, suggesting the absence of plagioclase and presence of garnet + amphibole in their residue. Considering a large gap among their crystallization ages, we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization. Mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition, and are metaluminous and enriched in LREE and LILEs, but are depleted in HFSE, and display an evolved Sr–Nd–Pb isotopic composition, suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere, which was metasomatized by adakitic melts and fluids from a subducted continental crust.In combination with the results of the Triassic ultra-high pressure metamorphic rocks in the Dabie orogenic belt, we apply a model involving the exhumation of subducted continental crust to explain the formation of the Wulong pluton. At the first stage, a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma, characterized by low SiO2 (60.65–63.98 wt.%) and high TiO2 (0.39–0.86 wt.%). The magma subsequently interacted with mantle peridotite, leading to high Mg# (57–67) and the metasomatism of the overriding mantle wedge. At the second stage, an asthenosphere upwelling that was probably caused by slab break-off at ca. 220 Ma melted the enriched sub-continental lithospheric mantle (SCLM) to produce mafic magmas, represented by the mafic enclaves that are hosted in the quartz and granodiorite, resulting in the partial melting of the shallower subducted crust, and generating the granodiorite that is distinguished by high SiO2 (69.16–70.82 wt.%), high Al2O3 (15.33–16.22 wt.%) and A/CNK values (mostly > 1.05). At the third stage, the final collapse of the Triassic Qinling–Dabie Orogenic Belt at ca. 215–205 Ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite, which is characterized by high SiO2 (67.68–70.29 wt.%), low TiO2 (mostly < 0.35 wt.%) and high Sr/Y ratios of 86–151.  相似文献   

15.
A comprehensive petrological and geochemical dataset is reported in order to define the thermo-compositional characteristics of Ti (Fe)-enriched picrite–basalt lavas (HT2, TiO2 3–7 wt%), erupted close to the axial zone of the inferred Afar mantle plume, at the centre of the originally continuous Ethiopian–Yemeni CFB plateau (ca. 30 Ma) which is zonally arranged with progressively lower Ti basalts (HT1, TiO2 2–4 wt%; LT, TiO2 1–3 wt%) toward the periphery. Integrated petrogenetic modelling based on major and trace element analyses of bulk rocks, minerals, and melt inclusions in olivines, as well as Sr–Nd–Pb–He–O isotope compositional variations enables us to make several conclusions. 1) The phase equilibria constraints indicate that HT2 primary picrites were generated at ca. 1570 °C mantle potential temperatures (Tp) in the pressure range 4–5 GPa whereas the HT1 and LT primary melts formed at shallower level (< 2–3 GPa, Tp 1530 °C for HT1 and 1430 °C for LT). Thus, the Afar plume head was a thermally and compositionally zoned melting region with maximum excess temperatures of 300–350 °C with respect to the ambient mantle. 2) The HT2 primary melts upwelled nearly adiabatically to the base of the continental crust (ca. 1 GPa) where fractionation of olivine, followed by clinopyroxene, led to variably differentiated picritic and basaltic magmas. 3) Trace element modelling requires that the primary HT2 melts were generated—either by fractional or batch melting (F 9–10%)—from a mixed garnet peridotite source (85%) with 15% eclogite (derived from transitional MORB protoliths included in Panafrican terranes) that has to be considered a specific Ti–Fe and incompatible element enriched component entrained by the Afar plume. 4) The LT, HT1, and HT2 lavas have 143Nd/144Nd = 0.5131–0.5128, whereas Sr–Pb isotopes are positively correlated with TiO2, varying from 87 Sr/86Sr 0.7032 and 206Pb/204Pb 18.2 in LT basalts to 87Sr/86Sr 0.7044 and 206Pb/204Pb 19.4 in HT2 picrite–basalts. High 3He/4He (15–20 RA) ratios are exclusively observed in HT2 lavas, confirming earlier evidence that these magmas require a component of deep mantle in addition to eclogite, while the LT basalts may more effectively reflect the signature of the pre-existing mantle domains. The comparison between high-MgO (13–22%) lavas from several Phanerozoic CFB provinces (Karoo, Paranà–Etendeka, Emeishan, Siberia, Deccan, North Atlantic Province) shows that they share extremely high mantle potential temperatures (Tp 1550–1700 °C) supporting the view that hot mantle plumes are favoured candidates for triggering many LIPs. However, the high incompatible element and isotopic variability of these high-MgO lavas (and associated CFB) suggest that plume thermal anomalies are not necessarily accompanied by significant and specific chemical effects, which depend on the nature of mantle materials recycled during the plume rise, as well as by the extent of related mantle enrichments (if any) on the pre-existing lithospheric section.  相似文献   

16.
The Xiaobaishitou W (–Mo) deposit is located in the eastern segment of the Central Tianshan, northwestern China. The deposit represents a skarn system distributed in the contact zones of biotite granite and crystalline limestone of the Mesoproterozoic Kawabulag Group. The Xiaobaishitou deposit is characterized by a typical calc-silicate mineralogy dominated by garnet, diopside and wollastonite, with minor epidote, tremolite, actinolite, chlorite, quartz, fluorite and calcite. The prograde and retrograde skarns are characterized by garnet–clinopyroxene–wollastonite and epidote–tremolite–actinolite–chlorite, respectively, intruded and replaced by mineral assemblages of scheelite–cassiterite–magnetite, quartz–sulfides and calcite–quartz–fluorite in younger order.Six molybdenite samples from the deposit yielded Re − Os isotope model ages ranging from 239.7 ± 3.6 Ma to 251.4 ± 3.6 Ma. The zircon crystals from biotite granite and Mo-mineralized granite yield weighted 206Pb/238U age of 242 ± 1.7 and 240.5 ± 2.1 Ma, respectively. Both the zircon U − Pb and the molybdenite Re − Os ages obtained in this study fall in a narrow span of 242–240 Ma, which suggest that the Xiaobaishitou W (–Mo) system was formed in the Triassic. The Re contents of the molybdenites range from 40.33 to 64.67 ppm, suggesting that the ore-forming materials were derived mainly from continental crust together with the involvement of minor mantle components. Combined with the 87Sr/86Sr ratios of tungsten-bearing quartz veins from other studies, which scatter between 0.707153 and 0.709877, demonstrating mixing between two end-member isotopic compositions of crust and mantle. It can be concluded that the Indosinian Xiaobaishitou deposit was formed in a tectonic transition from collisional crust shortening and thickening to post-collisional extension and thinning.  相似文献   

17.
Isotopic compositions of He (27 samples) and Ar (7 samples) as well as major and trace elements have been determined in Cenozoic effusive rocks and hosted olivine and pyroxene phenocrysts from northern and central Mongolia. The R = 3He/4He values are within (0.1–13)⋅10−6. Abnormally high R value, 13⋅10−6, atypical of Cenozoic basaltic-volcanism areas in Mongolia, has been first revealed at one of the sites of the Hangayn upland. In composition the rocks under study correspond to tephrites, trachybasalts, and subalkalic andesite-basalts. Analysis of their REE patterns and spidergrams shows that the elements participating in the formation of basalt fields of the Hangayn upland were supplied from the enriched mantle (EM1). These patterns are similar to the OIB ones; (La/Yb)n = 9–53. The R values in the olivine phenocrysts are higher as compared with the pyroxene phenocrysts and the bulk rock compositions of the same samples. Based on the elemental composition of the rocks, their contents of radiogenic 4He and 3He were calculated. The rate of 3He formation is 5.65⋅10−2 at/(g⋅year). The calculated and measured R values in the rock samples point to the presence of trapped mantle helium.  相似文献   

18.
Mafic granulite and spinel lherzolite xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr–Nd–Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal xenoliths using the two pyroxene geothermometer (T-Wells) yield 810–865 °C. The mineral assemblage (spinel–pyroxene–plagioclase) and Al content in pyroxene indicate that plagioclase-bearing xenoliths equilibrated at 5–8 kbar (13 and 30 km depth) in the lower crust. εNd and 87Sr/86Sr calculated at 700 Ma for Al-Ashkhara lower crustal xenoliths (+ 6.4 to + 6.6; 87Sr/86Sr = 0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle xenoliths consist of both dry and hydrous (phlogopite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt residues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are εNd = + 6.8 to + 7.8 and 87Sr/86Sr = 0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite xenoliths range between 0.59 and 0.65 Ga. The xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delamination replaced the E Arabian mantle root in Neoproterozoic time.  相似文献   

19.
The relationship among subducted oxidized oceanic crust and oxidation state of the subarc mantle, and arc magmas is one of the important aspects to evaluate convergent margin tectonics. However details of the oxidized mass transferred from buried oceanic crust to the overlying subarc mantle wedge remain obscure. Here we investigate the Songduo eclogites from south Tibet formed by the subduction of the paleo-Tethyan oceanic crust, and identify an abrupt decrease in pyrope and increase in almandine contents from the mantle to rim of garnet grains. This is coupled with a decrease in the Fe3 + content of epidote and Fe3 +/(Fe2 ++ Fe3 +) ratios from garnet core to rim domains, as well as speciation of calcite, a new mineral phase, in the rock matrix. Minor sulfates occur only as inclusions in garnet core domains, whereas sulfides are confined to the matrix as an accessory mineral phase. Aegirine augite occurs as relics or inclusions in garnet and omphacite. These features clearly suggest that oxidized components, Fe3 + and S6 +, were reduced as Fe2 + and S2 −, respectively, at the subduction zone. Thermodynamic modeling in the P–T-log10fO2 space using updated Perplex_X programs further revealed that the Songduo eclogites experienced oxygen fugacity variation of up to 8 log10 units, with decreasing pressure. Petrological observations further suggest that the strong redox processes took place, after breaking of garnet, during the initial exhumation of the eclogites. CO2 and minor sulfur are subsequently transferred from the cold oceanic subduction zone to the overlying mantle wedge, partially released by arc volcanoes to atmosphere. Our study presents a case of C and S recycling between the Earth's exterior and interior.  相似文献   

20.
Continental subduction and its interaction with overlying mantle wedge are recognized as fundamental solid earth processes, yet the dynamics of this system remains ambiguous. In order to get an insight into crust–mantle interaction triggered by partial melting of subudcted continental crust during its exhumation, we carried out a combined study of the Shidao alkaline complex from the Sulu ultrahigh pressure (UHP) terrane. The alkaline complex is composed of shoshonitic to ultrapotassic (K2O: 3.4–9.3 wt.%) gabbro, pyroxene syenite, amphibole syenite, quartz syenite, and granite. Field studies suggest that the mafic rocks are earlier than the felsic ones in sequence. LA-ICPMS zircon U–Pb dating on them gives Late Triassic ages of 214 ± 2 to 200 ± 3 Ma from mafic to felsic rocks. These ages are slightly younger than the Late Triassic ages (225–210 Ma) of the felsic melts from partial melting of the Sulu UHP terrane during exhumation. The alkaline rocks have wide ranges of SiO2 (49.7–76.7 wt.%), MgO (8.25–0.03 wt.%), Ni (126.0–0.07 ppm), and Cr (182.0–0.45 ppm) contents. The contents of MgO, total Fe2O3, CaO, TiO2 and P2O5 decrease with increasing SiO2 contents. The contents of Na2O, K2O, and Al2O3 increase from gabbro to amphibole syenite, and decrease from amphibole syenite to granite, respectively. The alkaline rocks have characteristics of an arc-like pattern in trace element distribution, e.g., enrichment of LREE, LILE (Rb and Ba), Th and U, depletion of HFSE (Nb, Ta, P and Ti), and positive Pb anomalies. From the mafic rocks to the felsic rocks, the (La/Yb)N ratios and the contents of the total REE, Sr and Ba decrease but the Rb contents increase. The alkaline rocks with high SiO2 contents also display features of an A2-type granitoids, e.g., high contents of total alkalis, Zr and Nb and high ratios of Fe2O3T/MgO, Ga/Al, Yb/Ta and Y/Nb, suggesting a post-collisional magmatism during exhumation of the Sulu UHP terrane. The alkaline rocks have homogeneous initial 87Sr/86Sr ratios (0.7058–0.7093) and negative εNd(t) values (− 18.6 to − 15.0) for whole-rock. The Sr–Nd isotopic data remain almost unchanged with varying SiO2 and MgO contents, suggesting a fractional crystallization (FC) process from the same parental magma. Our studies suggest a crust–mantle interaction in continental subduction interface as follows: (1) hydrous felsic melts from partial melting of subducted continental crust during its exhumation metasomatized the overlying mantle wedge to form a K-rich and amphibole-bearing mantle; (2) partial melting of the enriched lithospheric mantle generated the Late Triassic alkaline complex under a post-collisional setting; and (3) the alkaline magma experienced subsequent fractionational crystallization mainly dominated by olivine, clinopyroxene, plagioclase and alkali feldspar.  相似文献   

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